sd_bus_default() acquires a bus
connection object to the user bus when invoked in user context, or
to the system bus otherwise. The connection object is associated
with the calling thread. Each time the function is invoked from
the same thread, the same object is returned, but its reference
count is increased by one, as long as at least one reference is
kept. When the last reference to the connection is dropped (using
the
sd_bus_unref(3)
call), the connection is terminated. Note that the connection is
not automatically terminated when the associated thread ends. It
is important to drop the last reference to the bus connection
explicitly before the thread ends, as otherwise, the connection will
leak. Also, queued but unread or unwritten messages keep the
bus referenced, see below.

sd_bus_default_user() returns a user
bus connection object associated with the calling thread.
sd_bus_default_system() is similar, but
connects to the system bus. Note that
sd_bus_default() is identical to these two
calls, depending on the execution context.

sd_bus_open() creates a new,
independent bus connection to the user bus when invoked in user
context, or the system bus
otherwise. sd_bus_open_user() is similar, but
connects only to the user bus.
sd_bus_open_system() does the same, but
connects to the system bus. In contrast to
sd_bus_default(),
sd_bus_default_user(), and
sd_bus_default_system(), these calls return
new, independent connection objects that are not associated with
the invoking thread and are not shared between multiple
invocations. It is recommended to share connections per thread to
efficiently make use the available resources. Thus, it is
recommended to use sd_bus_default(),
sd_bus_default_user() and
sd_bus_default_system() to connect to the
user or system buses.

If the $DBUS_SESSION_BUS_ADDRESS environment
variable is set
(cf. environ(7)),
it will be used as the address of the user bus. This variable can
contain multiple addresses separated by ";". If
this variable is not set, a suitable default for the default user
D-Bus instance will be used.

If the $DBUS_SYSTEM_BUS_ADDRESS
environment variable is set, it will be used as the address of the
system bus. This variable uses the same syntax as
$DBUS_SESSION_BUS_ADDRESS. If this variable is
not set, a suitable default for the default system D-Bus instance
will be used.

sd_bus_open_system_remote() connects to
the system bus on the specified host using
ssh(1). host
consists of an optional user name followed by the
"@" symbol, and the hostname.

sd_bus_open_system_machine() connects
to the system bus in the specified machine,
where machine is the name of a local
container. See
machinectl(1)
for more information about the "machine" concept. Note that
connections into local containers are only available to privileged
processes at this time.

These calls allocate a bus connection object and initiate
the connection to a well-known bus of some form. An alternative to
using these high-level calls is to create an unconnected bus
object with
sd_bus_new(3)
and to connect it with
sd_bus_start(3).

The functions sd_bus_open(),
sd_bus_open_user(),
sd_bus_open_system(),
sd_bus_open_system_remote(), and
sd_bus_open_system_machine() return a new
connection object and the caller owns the sole reference. When not
needed anymore, this reference should be destroyed with
sd_bus_unref(3).

The functions sd_bus_default(),
sd_bus_default_user() and
sd_bus_default_system() do not necessarily
create a new object, but increase the connection reference of an
existing connection object by one. Use
sd_bus_unref(3)
to drop the reference.

Queued but unwritten/unread messages also keep a reference
to their bus connection object. For this reason, even if an
application dropped all references to a bus connection, it might
not get destroyed right away. Until all incoming queued
messages are read, and until all outgoing unwritten messages are
written, the bus object will stay
alive. sd_bus_flush() may be used to write
all outgoing queued messages so they drop their references. To
flush the unread incoming messages, use
sd_bus_close(), which will also close the bus
connection. When using the default bus logic, it is a good idea to
first invoke sd_bus_flush() followed by
sd_bus_close() when a thread or process
terminates, and thus its bus connection object should be
freed.

The life cycle of the default bus connection should be the
responsibility of the code that creates/owns the thread the
default bus connection object is associated with. Library code
should neither call sd_bus_flush() nor
sd_bus_close() on default bus objects unless
it does so in its own private, self-allocated thread. Library code
should not use the default bus object in other threads unless it
is clear that the program using it will life cycle the bus
connection object and flush and close it before exiting from the
thread. In libraries where it is not clear that the calling
program will life cycle the bus connection object, it is hence
recommended to use sd_bus_open_system()
instead of sd_bus_default_system() and
related calls.